1. Field of the Invention
The present invention relates to a fluorescence detection of a fine particle, and more particularly to a fluorescence detection of a base (nucleotide) forming a gene.
2. Description of the Prior Art
Deoxyribonucleic acid abbreviated to DNA is composed of bases as a primary component, sugar and phosphoric acid, and is in a double helix structure containing genetic information therein. The genetic information is present in each of the DNA fragments in the form of a sequence of bases. A strand of genes is found in the cell of an organism. For lower animals such as a microorganism, there are several thousand base pairs while for a higher level animal having much more genetic information, there are several hundred million to nearly three billion base pairs.
The genetic information of DNA is determined by a sequence of adenine (A), guanine (G), cytosine (C), and thymine (T) which are bases (nucleotides) forming a nucleic acid. Investigation of the sequence of these bases is therefore the key to develop gene engineering and medical science.
It has been known that each of the aforesaid bases produces specific fluorescence, and it has been practiced to identify the base depending upon the fluorescence produced. Production of fluorescence results from irradiation of exciting light onto the base. Detection of the fluorescence thus produced may preferably be performed using a high sensitivity detector such as photomultiplier tube.
A publication entitled "Chemical Physics Letters" (1990) 174, pages 552-557 discloses an arrangement for detecting fluorescence as shown in FIG. 1. An exciting light from a light source 62 is irradiated into a flow cell 61 wherein the direction C in which the exciting light is irradiated is perpendicular to the direction A.sub.1 in which fine particles are supplied to the flow cell 61. The fluorescence emitting in the direction of D is detected by a photomultiplier tube 63, wherein the direction D is perpendicular to both the direction A.sub.1 and the excited light irradiation direction C.
The arrangement as above is, however, incapable of accurately and efficiently detecting the fluorescence from the fine particle such as the base of gene. A single DNA fragment has an extremely large number of bases, and thus each base is extremely small in size, and the fluorescence produced from each base is extremely minute. It is therefore difficult to identify the kind of the base with the detection of the fluorescence which takes place for an ultra short period of time when the exciting light beam passes through the fine particle.
While it it is possible to prolong a period of time during which the fluorescence is produced and enhance the efficiency of fluorescence detection if the exciting light beam is irradiated over the streamwise range of the fine particles, a problem arises that processing of the data collected is difficult because sequentially supplied fine particles are detected one at a time. The conventional arrangement has a further disadvantage that only the fluorescence produced in one direction can be detected, and thus it is low in efficiency.